Electric furnaces are classified into alternating current electric furnaces and direct current electric furnaces. In an alternating current electric furnace, three graphite electrodes are inserted from above into the furnace to form arcs between those electrodes mainly through scraps or molten steel. In a direct current electric furnace, usually one graphite electrode is inserted into the furnace and direct current arcs are allowed to form between the graphite electrode and the furnace bottom as the other electrode.
In the former, since three AC electrodes are used, the upper structure of the furnace is complicated and three-phase arcs are deflected outwards by a mutual electromagnetic force, so that the quantity of radiant heat is large and the thermal efficiency is poor. Besides, the furnace wall is damaged locally due to the deflection of arc. Further, the wear of the electrodes is conspicuous, the noise is loud, and flicker is marked. On the other hand, in the latter, i.e., a DC electric furnace, the construction around the furnace top electrode is simple because the number of electrode used is generally small, and in comparison with an AC electric furnace, the decrease in unit consumption of the graphite electrode and in electric power unit consumption as well as the decrease of flicker can be expected. However, problems are encountered in the service life and safety of the furnace bottom electrode.
As stated in an article entitled "Chokuryu Kanetsuro No Genjyo To Shyorai" ("Present State and Future of DC Arc Furnaces") at page 24-33 of "Kogyo Kanetsu Ro" ("Industrial Heating Furnaces") Vol. 25 (1988), No. 2, published by Nippon Kogyo Ro Kyokai (Japan Industrial Furnace Association), as furnace bottom electrodes in DC electric furnaces there are known a small-diameter multi-electrode air-cooled type having a large number of small-diameter electrodes embedded upright in a refractory provided as a lining on the furnace bottoms and a large-diameter electrode water-cooled type having one to three round steel rods of a large diameter disposed upright in the furnace bottom.
FIG. 6 shows an example of a conventional DC electric furnace of a large-diameter electrode type. In the same figure, three top electrodes 118 made of graphite are inserted into the furnace through a furnace lid 112, while in a furnace bottom 116 three water-cooled type bottom electrodes 130 constituted by steel rods are embedded upright in a molded refractory. The diameter of each bottom electrode 130 is about 250 mm at most. Thyristors 124 constitute electrode control circuits which are each independent together with the three top electrodes 118 and the three bottom electrodes opposed thereto, to control voltage and current. Under this construction, when the total capacity of furnace transformers is 60 MVA, each transformer controls an applied electric power in the range of 20 MVA, so in a steady state there are formed three arcs.
In a DC electric furnace of such a large-diameter bottom electrode type when all of the three bottom electrodes 130 cease to conduct due to the adhesion of slag to the electrodes, one of the top electrodes 118 is connected to an anode side to form arcs between the top electrodes, whereby the scraps in the furnace can be melted. In this case, although the scrap melting time becomes longer because of decrease of the applied electric power, it is possible to cope with the non-conduction of electric current relatively easily.
On the other hand, in the large-diameter electrode type shown in FIG. 6, since three top electrodes 118 are used as in the conventional AC electric furnaces, it is necessary to provide three systems with respect to all of electrode supporting arms, electrode lift devices and electrode control circuits, resulting in that the equipment is complicated and the equipment cost and maintenance cost are increased inevitably. Moreover, as shown in FIG. 7, since the positions of the three top electrodes 118 in a furnace body 110 are asymmetric with respect to the furnace wall, cold and hot spots are formed on the furnace wall, thus impeding uniform melting of the scraps. Further, a small ceiling 112a of the furnace wall 112 is damaged in an extremely early stage due to radiant heat or arcs. Additionally, since there occurs an unmelted scrap portion A in each cold spot, an extra electric power is required to melt the unmelted portion A, with the result that the required time from tap to tap in withdrawing molten steel from the furnace is extended and, so the unit consumption of each of electric power, electrodes and refractory is increased, thus leading to increase of the cost.
Another background art will be described below.
FIG. 8 is a schematic sectional view of a conventional DC electric furnace of a small-diameter multi-electrode air-cooled type. A further body 10 of this furnace is composed of a furnace lid 12, a furnace wall 14 and a furnace bottom 16. One (two or three as case may be) graphite electrode 18 is inserted into the furnace body through the furnace wall 12, and a water cooling panel 20 is attached to the furnace wall 14. In one end portion of the furnace bottom 16 there is formed a tapping hole 24 for molten steel after refining, while in an opposite end portion of the furnace bottom there is formed a slag-off hole 22 for the discharge of slag. Further, a large number of furnace bottom electrodes 30 each constituted by a steel rod of a small diameter are embedded in the furnace bottom 16, and the furnace body 10 can be tilted right and left by a tilting device (not shown) such as a hydraulic cylinder for example. A tap flap 26 for blockading the tapping hole 24 is disposed for opening and closing motions just under the hole 24.
As the bottom electrodes 30 which are a large number of small-diameter electrodes embedded in the furnace bottom, for example in a furnace having a capacity of 130 t/heat, a large number (200 or so) of round steel rods each about 40 mm in diameter are embedded upright in a refractory 28' lined by stamp on the furnace bottom 16. The bottom electrodes 30 form an anode in an electrode circuit, while the graphite electrode 18 projecting from the furnace lid 12 is opposed as a cathode to the anode. In this case, a maximum diameter of each bottom electrode 30 is about 40 mm.
As shown in FIGS. 9 and 10, the stamp material 28' is stamped around the bottom electrodes 30, and the upper end faces of the electrodes 30 are exposed to the upper surface of the stamp material 28' while the lower ends thereof reach an electrode supporting plate 32 projecting to the exterior of the furnace and spaced from a bottom plate 16a and is fixed with clamping nuts 7. Cooling air is supplied between the electrode supporting plate 32 and the bottom plate 16a from a cooling air pipe 34 formed of an electroconductive material and connected to the electrode supporting plate 32, thereby cooling the lower portions of the bottom electrodes 30. Usually, the stamp material 28' on the bottom plate 16a and the cooling plate (electrode supporting plate) 32 are constituted integrally with the bottom electrodes 30 and these can be replaced as a block. The numeral 4 denotes an insulator for insulation between the bottom plate 16a and a furnace bottom shell 16', and the numeral 5 denotes a power supply cable, which is a water-cooled type. In a molten steel forming stage, an electric current is supplied along the following route: power supply cable 5.fwdarw. cooling air pipe 34.fwdarw. electrode supporting plate 32.fwdarw. bottom electrodes 30.fwdarw. molten steels scraps .fwdarw. top graphite electrode 18.
As shown in FIG. 8, electric power is supplied through a receiving transformer 21 in a power supply circuit and is fed to thyristors 25 after the voltage thereof is transformed to 200-800 V by transformers 23 for the furnace. The thyristors 25 are provided in a single system of an electrode control circuit which connect the top electrode 18 and the bottom electrodes 30 with each other, and thus the control of melting in the DC electric furnace is performed by a single system. Voltage control is performed by a positional control for the graphite electrode 18 through an electrode lift mechanism (not shown), while current control is performed by control of the thyristors 25.
Thus, because of a single graphite electrode 18, the construction around the top electrode is simple and the decrease in unit consumption of the graphite electrode 18 and in electric power unit consumption can be expected. Besides, control is easy because the control of melting in the DC electric furnace can be done by a single system.
In the DC electric furnace of an air cooling type shown in FIG. 8, having a large number of round steel rods as the bottom electrodes 30 embedded in the furnace bottom, for example when the furnace capacity is 130 t/heat, there are used about 200 such round steel rods each having a diameter of up to 40 mm or so. These many bottom electrodes 30 are embedded in the stamp refractory 28' and all of them are connected to a single electrode supporting plate 32 so that an electric current is supplied to all the bottom electrodes 30 at a time from a single power cable 5 from the electrode supporting plate 32. This DC electric furnace shown in FIG. 8 involves the following problems based on its construction and air cooling type.
(1) With repeated charge for melting and refining of scraps using direct current, the bottom electrodes of a small diameter are melted by both the heat from molten steel and Joule heat generated by the internal electric current, but because of an air-cooled type, not a water-cooled type, the heat removing ability is low and a limit is encountered in enlarging the diameter of the bottom electrodes. An upper limit of the diameter is 40 mm or so. PA0 (2) It is impossible to make a fine electric current control for the bottom electrodes because electric power is supplied to a large number of bottom electrodes at a time. PA0 (3) Slag is more likely to adhere to the upper portions of the electrodes because of a large number of the electrodes, and once electrodes cease to conduct an electric current, an excessive current flows through the other bottom electrodes in the case where the supply current is constant, resulting in that the furnace operation is badly influenced. PA0 (4) Because the number of the bottom electrodes is too large it is actually impossible to monitor the melting condition of each electrode using a thermocouple. PA0 (5) Due to correlation of the above problems (1)-(4) an average electric current density per bottom electrode is only about one half of that in the water cooling type and thus the efficiency is poor. PA0 (6) Since the number of the bottom electrodes is large, it is only a stamp refractory that can be applied between the electrodes. Consequently, in comparison with the brick refractory, the rate of damage and wear is high and the service life of the bottom electrodes is short. PA0 (7) The directionality of arcs generated in the furnace is influenced by a magnetic field created around the power cable which supplies an electric current to the electrodes. Since an electric current is fed to a large number of bottom electrodes simultaneously through a single power supply cable, the direction of arc is determined by the arrangement of the power supply system, so it is impossible to diminish hot and cold spots.